A simple and efficient procedure for incorporating the effects of stream channel geometry in the distributed modeling of catchment dynamics is developed. At-a-station and downstream fluvial relationships are combined and the obtained laws of variability in space and time for water-surface width and wetted perimeter are incorporated into a diffusion wave routing model based on the Muskingum-Cunge method with variable parameters. The parameterization obtained is applied to the approximately 840-km2 Sieve catchment (Central Italian Apennines) to test the possibility of estimating channel geometry parameters from cross-section surveys and to assess the impact of dynamic variations in the channel geometry on catchment dynamics. The use of the estimated channel geometry in surface runoff routing produces a significant improvement in the flood hydrograph description at the catchment outlet with respect to less detailed network parameterizations. In addition, the results obtained from a downstream analysis of the velocity field indicate that the stream characteristics related to the locally varying cross-section shape may have a strong control on flow velocities, and thus they should be monitored and synthesized for a comprehensive description of the distributed catchment dynamics. ¿ 1998 American Geophysical Union